1. Field of the Invention
The subject invention generally pertains to electronic power conversion circuits, and more specifically to high frequency switched mode power electronic converter circuits.
2. Description of Related Art
There are some power conversion circuits which accomplish higher efficiencies by implementing a mechanism that accomplishes switching at zero voltage. Power loss in a switch is the product of the voltage applied across the switch and the current flowing through the switch. In a switching power converter when the switch is in the on state the voltage across the switch is zero so the power loss is zero. When the switch is in the off state the power loss is zero because the current through the switch is zero. During the transition from on to off, and vice versa, power losses can occur, if there is no mechanism to switch at zero voltage or zero current. During the switching transitions, energy losses will occur if there is simultaneously (1) non-zero voltage applied across the switch and (2) non-zero current flowing through the switch. The power losses associated with the switching transitions will be the product of the energy lost per transition and the switching frequency. The power losses that occur because of these transitions are referred to as switching losses by those people who are skilled in the art of switching power converter design. In zero voltage switching converters the zero voltage turn off transition is accomplished by turning off a switch in parallel with a capacitor and a diode. The capacitor maintains the applied voltage at zero across the switch as the current through the switch falls to zero. In the zero voltage transition the current in the switch is transferred to the parallel capacitor as the switch turns off.
The zero voltage turn on transition is accomplished by discharging the parallel capacitor using the energy stored in a magnetic circuit element such as a transformer or inductor and turning on the switch after the parallel diode has begun to conduct. During the turn on transition the voltage across the switch is held at zero clamped by the parallel diode. The various zero voltage switching techniques differ in the control and modulation schemes used to accomplish regulation and in the energy storage mechanisms used to accomplish the zero voltage turn on transition.
One of the techniques uses a resonant circuit which is frequency modulated over a broad frequency range. An example is shown in FIG. 1. These techniques have been refined by a multi-resonant technique in which more resonant circuit elements and a complex control circuit are required but the converter can operate at a fixed frequency.
Several techniques have been introduced which accomplish zero voltage switching inherently at constant switching frequency. One of these techniques requires a full bridge switching arrangement with four primary switches in which the regulation is accomplished by phase modulation. This technique is illustrated in FIG. 2. This technique has several drawbacks including the limited availability of phase modulated integrated control circuits and the large number of parts, which include four primary switches, at least two secondary switches, and at least two large magnetic circuit elements. The technique suffers from an inability to accomplish zero voltage switching at light loads without additional circuit elements and additional complexity.
Another circuit based on the single ended forward converter accomplishes zero voltage switching by addition of an additional primary side switch and capacitor. This technique is illustrated in FIG. 3. This converter's disadvantages are the additional voltage stress on the primary switching elements required to reset the transformer core and a high parts count. The parts required are two large magnetic circuit elements, the transformer and the output filter inductor, two primary switches, a large primary capacitor, and two secondary switching elements.
There is one example of prior art that accomplishes zero voltage switching and a low component parts count. This circuit, shown in FIG. 4, relies on a single magnetic circuit element which accomplishes both magnetic energy storage and isolation. This converter relies on high AC magnetizing fields to accomplish zero voltage switching by requiring that the magnetizing field and the magnetizing current change sign during each cycle. To accomplish zero voltage switching in the circuit of FIG. 4 the peak-to-peak AC magnetizing current must be greater than twice the maximum load DC magnetizing current and the peak-to-peak AC magnetic field must be greater than twice the maximum load DC magnetic field. The high AC magnetizing fields create high core losses at high switching frequencies. The high AC magnetizing currents result in high peak currents and high associated conduction losses. The primary motivations for zero voltage switching are to obtain higher efficiency, higher operating frequencies, smaller component sizes, and higher power densities. Increasing the AC magnetizing currents reduces semiconductor switching losses but increases core and conduction losses, These increased losses impose a limit on the level of power density and efficiency that can be obtained with this approach.